Flue gas quenching and acid gas removal (especially removal of SOx and NOx) from flue gases is often performed in a single quench reactor/contactor in which the hot flue gas is contacted with an aqueous solution of sodium hydroxide to thereby reduce temperature and the acid gas content in the flue gas. Such flue gas treatment is conceptually relatively simple and effective for SOx and NOx removal, however, several problems remain. Among other things, the caustic and loaded aqueous solution typically requires treatment and/or other processing steps before the water from these solutions can be released into the sewer system without negative environmental impact. Moreover, SO2 content of less than 50 ppm in the treated gas from such devices is typically not achieved.
Other known flue gas treatment configurations may employ dual or multiple reactors in which separate reactions are performed as described, e.g., in U.S. Pat. No. 4,400,362. Here, a first reactor is operated as sulfurous acid conversion scrubber that also generates reactive nitrogen oxides (e.g., N2O3), which are then absorbed in a second stage by an alkali metal ammonium salt or hydroxide using complex reaction kinetics between SO2 and NOx in water in the presence of O2. Typically, the solvents produced by such systems are not suitable for discharge to sewer or re-use in facility, and the vented gas will typically have a SOx level of above 50-100 ppm. Similar problems are encountered in U.S. Pat. No. 3,920,421 in which water is used as the sorbent for NOx and SOx.
In further known configurations, as shown in U.S. Pat. No. 5,607,654, the incinerator gases from a medical waste treatment plant are first contacted with a dry reagent to thereby immobilize/convert acid gases on a dry solid that is subsequently removed in baghouse or electrostatic precipitator. So treated gas is then quenched in a quench pipe and subsequently scrubbed using acid liquor to remove toxic organic compounds. Water from the treated gas and the quench step is carried over to the scrubber, and excess liquor is routed back to the incinerator. While such configurations eliminate at least in some configurations the need for waste water purification, substantial quantities of water are nevertheless required. Moreover, as the quench water is carried over to the scrubbing stage, scrubbing solvent is continuously diluted and must be replenished. Worse yet, due to the dilution of the scrubbing solvent, the absorption capacity of SOx and other contaminants is reduced.
Alternatively, as described in U.S. Pat. No. 5,154,734, waste gas is routed through a quencher to saturate the waste gas with water. So loaded waste gas is then fed to a wet scrubber in which the water in the waste gas is condensed to thereby enhance fine particle, acid gas and heavy metal recovery. Further removal of the contaminants leaving the scrubber in the treated gas is then achieved using a collision scrubber/entrainment separator. While such systems provide at least some advantages, various disadvantages nevertheless remain. Among other things, the quantity of circulating water (e.g., between wet scrubber and quench section) is relatively large. Furthermore, the contaminant laden water can not be discharged into the sewer system but must be regenerated in a separate facility.
In yet another configuration, flue gas is first contacted with sulfite or bisulfite to thus reduce the SO3 and/or hydrogen halide concentration in the flue gas. Remaining SO2 is then removed in a wet scrubber (a typical configuration is described in U.S. Pat. No. 6,126,910). While such systems provide some improvements (e.g., reduction of downstream components) other difficulties remain. For example, SO3 removal from the pretreated stream is performed in a single stage quench scrubber.
Thus, while numerous configurations and methods of flue gas treatment are known in the art, all or almost all of them, suffer from one or more disadvantages. Therefore, there is still a need for improved configurations and methods of flue gas treatments.